Micro-electromechanical pressure sensor systems (MEMS) are well known. They are formed of a thin silicon diaphragm supported by a relatively thick, relatively massive substrate. The diaphragm flexes responsive to pressure applied to the diaphragm through a port. A piezo-resistive Wheatstone bridge circuit formed into the diaphragm, changes its resistance responsive to diaphragm deflection.
MEMS sensors are usually attached to a surface of a housing or substrate that is to be attached to a component of a vehicle and used to sense various pressures. MEMS sensors are typically fixedly attached to housing in order to be made operable. The die on which the MEMS sensor is formed, also seals the pressure port and should have enough strength to prevent bursting.
There is usually a large difference between the thermal expansion coefficient (CTE) of a MEMS device and the housing for the MEMS device, whether it is plastic or ceramic. When the MEMS device and housing are bonded together with an adhesive, the bonding stress induced by the CTE differences can affect the MEMS output.
Low modulus silicone is widely used to reduce bonding stress for low pressure sensors, which also should be able to prevent bursting when subjected to three or more times the maximum operating pressure. Low modulus die bonding material, however, is usually weak and therefore not well-suited for use with medium pressure sensors, e.g., transmission fluid pressure sensors, engine oil sensors, and the like. A stronger bonding material is needed for medium pressure sensors. But the bonding stress typically increases with their usually higher moduli. Increasing a bonding area can increase a bond's strength, however, increasing the bonding area is often not practical or feasible due to the fact that the size of MEMS sensors are shrinking for at least cost-reduction reasons (or physical constraints for packaging). Bonding stress and bonding strength therefore should be carefully considered. Matching the thermal expansion coefficients of a silicon MEMS sensor device and the use of lower modulus polymer based housing and a mounting adhesive would be an improvement over the prior art.